This invention relates to a method and a circuit for exciting an ultrasonic generator and the use thereof for atomizing a liquid.
The possibility of atomizing liquids with the aid of piezoelectric ultrasonic generators is known. For example, the article by W. D. Drews "Flussigkeitszerstaubung durch Ultraschall" ("Liquid Atomization by Ultrasonic Energy") in "Elektronik" (1979), No. 10, pp. 83-90 briefly describes the principle of this method. An ultrasonic generator is utilized which is equipped with an atomizer disk or plate and a circuit for exciting this ultrasonic generator.
However, the technical realization of the atomization of a liquid using an ultrasonic generator has been difficult due to a number of problems.
An atomization is only possible close to the resonance of an ultrasonic generator (together with its atomizer disk), and the necessary exciting frequency must be very precisely maintained. The locking of the oscillator of the exciting circuit to an apparent resonance, which does not correspond to an effective atomization must be reliably prevented.
The exciting circuit must be in a position to detect changes in the necessary exciting frequency as a function of different parameters. Such parameters are e.g. the manufacturing tolerances of the mechanical components of the ultrasonic generator (particularly its atomizer disk), the variations in the mechanical and electrical parameters of the piezoelectric ceramic used in its manufacture, the operating temperature of the ultrasonic generator (very important when used in burners), the aging of the ultrasonic generator, deposits formed thereon (such as e.g. soot and resins when used in burners) and the manufacturing adjustment and other tolerances in the exciting circuit.
Reliable detection of a stoppage of atomization must be ensured. If stoppage is caused by droplets which have stuck to the atomizer disk, the centrifuging of these droplets from the disk must be ensured.
A practical requirement with respect to industrial use is the interchangeability of the exciting circuit and the ultrasonic generator itself or optionally its atomizer disk and namely without any matching or adapting and without high tolerance requirements on the replacement parts of spares.
To achieve the best possible efficiency the atomizing capacity of the ultrasonic generator or its atomizer disk must be automatically regulatable, without any action by an operator and without having to change e.g. the exciting voltage or the duty cycle of the drive frequency.
Numerous methods and circuits have already been proposed for solving these problems.
DE-3222425 proposes exciting the ultrasonic generator across a matching network, which inter alia serves to suppress the starting of oscillation of the ultrasonic generator to harmonics of its resonant frequency. The direct current component of the resonator current is used for regulating the exciting current and the alternating current component of the resonator current is used for regulating the exciting frequency, a band pass filter only permitting the passage of the frequency component at the desired resonant frequency of the ultrasonic generator. In the case of a resonance failure the exciting frequency is wobbled or swept, in order to pass through the resonance point and to obtain relocking. It is a disadvantage of the solution that the circuit is matched to the ultrasonic generator and particularly to its desired resonant frequency, so that the operation of the ultrasonic generator cannot follow the changes in certain of the aforementioned parameters and also the easy interchangeability of components is not ensured. A reliable operation is not ensured in the case of oscillation starting, particularly under load and with varying operating conditions, because the impedance and therefore the phase relationships between the current and the voltage of the ultrasonic generator vary considerably in the case of load changes and consequently it is not possible to track the optimum oscillating frequency, derived from the phase relationship between the current and the voltage in the ultrasonic generator. A true compensation of the capacitance of the ultrasonic generator by means of its inductance is not possible due to the capacitance changing during load changes.
With a somewhat different construction much the same is proposed in U.S. Pat. No. 4,275,363, wherein the same, aforementioned disadvantages occur.
DE-3314609 proposes operating the ultrasonic generator with timed bursts using different values thereof in each case. However, it is disadvantageous for the frequency matching and the control of the bursts to use the free dying out of an oscillation instead of the resonance behavior of the ultrasonic generator, because then it is not possible to obtain values varying in linear manner with the actual state.
In a somewhat different construction much the same is proposed in DE-3401735 wherein, also, the same, aforementioned disadvantages occur.
DE-3534853 proposes operating the ultrasonic generator with timed bursts and to carry out a current measurement during specific times for automatic frequency matching purposes. The necessary intermediate storage of the current measurement value and the precise synchronization of the measurement and control sequences are disadvantageous and, in particular, costly.